1. Field of the Invention
The present invention relates in general to a sealoff device and method for controlling the level of a fluid within first and second confined regions arranged in fluid communication with one another, and more particularly, to such a device and method adapted to shielding, from nuclear radiation, a tube bundle contained within the wrapper of a nuclear steam generator by controlling the level of a shielding media, i.e., water, within the interior of the wrapper and while maintaining the annular downcomer passage free of the shielding media above a predetermined elevation to permit dry access therein.
2. Description of the Prior Art
The generation of electrical power using nuclear energy requires the use of a steam generator for conversion of a working fluid, i.e., water, into pressurized steam. The steam generator is generally constructed of a tube bundle having a plurality of heat transfer tubes extending from a supporting tube sheet and contained within a cylindrical wrapper which defines an annular downcomer passage between a concentric outer shell. A mixture of fresh feed water and return water from a liquid/vapor separator enters the tube bundle area through openings in the wrapper at the end of the annular downcomer passage near the tubesheet. This fluid is on the outside of the tubes and is not normally contaminated. Particular radiation emitting contaminants are deposited on the inside of the tubes from primary water that is heated in the reactor core. These primary system contaminates give off a relatively high radiation field. The annular downcomer passage between the outer shell and wrapper of the nuclear steam generator is accordingly subjected to such a high radiation field as a result of these deposited radioactive contaminants.
During the operating life of the steam generator, it is periodically required that access be gained to the annular downcomer passage for internal repair and/or maintenance. In order for an operator to work within the area of the annular downcomer passage for long periods without exceeding relatively low government radiation limits, it is required that the tube bundle within the wrapper be effectively shielded by, for example, raising the water level to entirely cover the tube bundle. As is well-known, water provides one of the more effective shielding media for nuclear radiation. However, as the water level rises over the tube bundle, so does the water level within the annular downcomer passage as a result of their being constructed in fluid communication with one another. The performing of work of any nature within the annular downcomer passage while it is under a high water level condition, creates problems of safety to the operator and inaccessibility to the lower regions of the downcomer passage. If the water level is lowered in the annular downcomer passageway to increase accessibility and to permit work to be performed in a dry environment, the water level about the tube bundle is lowered a corresponding amount. As a consequence, a portion of the tube bundle becomes unshielded, causing the work area to be subjected to a high radiation field. This high radiation field created in the work area will decrease the work time allowed within the limits of allowable government radiation exposure for these workers.
Accordingly, it can be appreciated that there is an unsolved need for a sealoff device and method which permits the maintaining of a high water level inside the wrapper of a steam generator for shielding workers from the radiation source contained tube bundle, while providing a dry work area within the annular downcomer passage, which in addition to being greatly reduced in exposure to the radiation field, provides for greater accessibility to the exterior of the wrapper for repair and maintenance.